Estimating MP with Heterochromatic Flicker Photometry
The HFP technique is based to the fact that MP acts as a filter which effects the sensitivity to different wavelengths. It absorbs blue light more than green.
Two superimposed lights alternately going on and off will appear to flicker unless they are at the same luminance
The subject presses a button when flicker becomes visible
The blue / green ratio is changed after each button press.
As the blue/green luminances get closer, flicker sensitivity is reduced.
This produces the graph which shows a minimum at isoluminance.
The difference between the minimum for central viewing and for peripheral viewing provides a measure of Macular Pigment density in the central fovea.